How about that white plastic stuff, forget the name. The places I see it they charge a fortune for it. I'd like to use it instead of wood myself but the cost is ridicules.
That's what I used on my last sled. I purchased it at a woodworking supply store, it was precut for use on a miter sled! Here's is my take on it from that experience with a single miter sled:
1) It is very flexible, and relies on a shallow dado in the sled to insure that it is straight and true.
2) The plastic miter bar was exactly the same width as my table saw miter slot -- the fit was too tight. I tried to sand the sides of the plastic bar, but this did not seem to work.
3) I drilled and tapped the miter track, and used flathead machine screws every 4-5 inches. If the screw were too tight, the sled would bind.
4) The solution to my problem was to run the plastic miter bar through my planner. I used a sled under the bar to keep it flat.
In conclusion, I think the material is stable, smooth, and could be used to make good miter bars, but not the way I did it.
The miter bar I do like is an all metal bar with ball bearing adjusters in one side. I purchased it from WoodCraft several years ago! Only problem is, they were only available in 18" lenth at that time; not really long enough for a large sled!
I used straight-grain Oak for the sled I made last month. It works good for the time being, time will tell!